I tend to disagree....
Since with node blockades you KNOW where the enemy will come from your beam cannon willl not have to move much. Given the re-charge times of beam cannons, cruiser type mauverability is more than enough.
Firstly, we don't know where the enemy will come from. There's no conclusive evidence that the exit vector from subspace is fixed. Secondly, the beam cannons need to manuever, fire, and inflict disabling damage in the time period before an attacking ship can jump out. That's not a long time; not to mention that an attacking destroyer (or fighter-carrying carrier) will be able to deploy fighters as cover (whilst a sensible blockade will have protective fighter cover of its own, it's best to make sure the Mjolnirs have a good chance of hitting).
Note, again, Mjolnirs are not perfect in this regard. But they are, most likely (due to the fact they were developed) cheap and effective for their cost-loss ratio. It's probably a LOT cheaper to position a reasonable number of Mjolnirs in a good 360 firing arc round a node, than to have the same coverage via capital ships. This does not abdicate other ship types from a role in a blockade, of course.
RBC's aren't cheap. If they were we would be seeing many more of them. Giving them the abiltiy to turn around and jump out means you can save them and re-use them. It's efficent from any point of view, as you can mount a even stronger a defense at the next node.
Actually, we don't see many because they are developed fairly late on in the campaign and marked as experimental. After that point in the game you don't see many blockades, period.
And techincly, hte GTVA could hold a blocake quite well with the tech it has.. Put several RBC'ss, Orions and Hatephsuts around the node and I guarnatee that no Sathanas will make it trough. A jump node is a tight spot where only one destroyer/jugg can pass trough at once.
The GTVA tried that in Capella. The Sathanas decimated them;
[A small task force will oversee the demolition of the Knossos while the bulk of the allied fleet blockades the jump node leading to the densely-populated Capella system. Command has begun the process of evacuating the two hundred fifty million civilians inhabiting Capella, the largest exodus since the Great War. The Colossus will remain in that system to engage the Sathanas should we fail.
The Sathanas decimated our fleet at the Capella node. Your bomber wing will be scrambled to destroy the Juggernaut's forward beam turrets before it reaches Capella. You must destroy at least two turrets to give the Colossus a fighting chance. You will be armed with the new long-range Trebuchet missile and the Helios torpedo, the GTVA's most powerful tactical warhead.
Either the bulk of the GTVA fleet is smaller than several RBCs and multiple destroyers (meaning that strategy would be unfeasibly resource-intensive with the risk of Shivan attacks from other unstable nodes), or it was that size and simply annihilated by the superior vessel. If the storyline of FS2 shows one thing about blockades, it's that they are frequently broken. Also, there's a potential issue that beam cannons overheat with continuous fire on a capship, i.e. the Colossus. I'd imagine Mjolnirs would be designed from scratch to avoid this.
Whilst I would agree RBCs with fighter cover and warship (but not primarily destroyers; you need to keep them at a safe distance or you risk losing your entire fighter and bomber cover) cover in the perimeter, you can't hold out against a vastly superior force under any prolonged circumstance.